These lines are a prophesy by Apollo to the father of Cydippe. I want these lines to mean something like, "For I say that Acontius will be mingled with you not as lead with silver, but as amber with gleaming gold." However, isn't μειξέμεναι the future active infinitive, and , if the preceding translation is correct, shouldn't σε be in the dative to correspond with ἀργύρῳ and χρυσῷ? If we do take μειξέμεναι as the future active infinitive, I don't understand with what the σε is being mixed.

I think that σε is the subject [indirect discourse], the father, and Acontius is the direct object of the verb, with implied indirect object, Cydippe. Middle-liddell has μ. τί τινι, to mix one thing with another.

"I say that you will join Acontius [with her] not as lead with silver, but as amber with gleaming gold."